“Our Rabbis taught: The Amen uttered in response [to the benediction before a meal] should be neither hurried nor curtailed nor orphaned, nor should one hurl the blessing, as it were, out of his mouth.”
That is to say, saying “Amen” is like saying the blessing itself. It should not be rushed or slurred - for example by missing the first letter, or cut short - by not pronouncing the last letter. It should not be said by someone who didn’t actually hear the blessing, but is only hearing other people saying “Amen.” Nor should it be a “throw away.”
We take our prayers seriously, we take our responses seriously.